1. Field of the Invention
The present invention relates to an optical head apparatus that performs recording or reproduction of optical information recording media.
2. Description of the Prior Art
Generally, a rewritable type optical disc must monitor the quantity of incident light to the recording surface of the disc to secure the signal recording quality with high accuracy. For this reason, the accuracy of a system that monitors the light quantity using light output from the posterior end face of a laser chip used in a reproduction-only optical head is not high, and therefore it is necessary to monitor the light quantity using light radiated from the anterior end face of the laser chip (hereinafter referred to as “anterior light”).
On the other hand, while optical discs are attracting attention as large-capacity information memories, optical head apparatuses need to attend a demand for high-speed recording or reproduction of optical discs. To meet this demand, it is necessary to increase the speed of modulation of a semiconductor laser light source and at the same time improve responsivity of the above described monitoring of the anterior light.
A conventional optical pick up will be explained with reference to the attached drawings below. FIG. 14 shows an example of an outlined configuration of a conventional optical head apparatus. A diverging beam 802 radiated from a semiconductor laser light source 801 passes through a parallel flat plate 803 placed diagonally to the optical axis and is converted to a parallel beam 805 by a collimate lens 804.
This collimated beam 805 is partially reflected by a polarized beam splitter 806 and enters into a photodetector 809. A beam 810, the major portion of the collimated beam 805, passes through the polarized beam splitter 806 and is converted to a circularly polarized beam by a ¼ wavelength plate 811, and then condensed into an optical disc 814 through an objective lens 813 mounted on an actuator 812.
The beam reflected by the optical disc 814 passes through the objective lens 813 and is converted by the ¼ wavelength plate 811 to a linearly polarized beam, which is orthogonal to the polarization plane of the outgoing radiation beam of the semiconductor laser light source 801 and entered into the polarized beam splitter 806.
Since the polarization plane of the incident beam entered into the polarized beam splitter 806 is orthogonal to the first half of the optical path, the incident beam is reflected by the polarized beam splitter 806, diffracted by a hologram element 815, branched into a positive 1st-order diffracted light 817 and negative 1st-order diffracted light 818 with the optical axis of the incident light as an axis of symmetry, then condensed by a detection lens 817, entered into signal detectors 820 and 821, respectively, to detect control signals such as focusing and tacking, and RF signals.
On the other hand, photodetector 809 that detects light reflected by the polarized beam splitter 806 acts as an output light quantity monitor of the semiconductor laser light source 801.
Here, the reason why the parallel plate 803 is placed diagonally to the optical axis of the incident beam between the semiconductor laser light source 801 and collimate lens 804 will be explained. Generally, as for a semiconductor laser used for a light source of the optical head apparatus, from the standpoint of an optical characteristic, mode west of an oscillated beam of a semiconductor laser element 901 differs between the semiconductor composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane) as shown in FIG. 15.
That is, while the mode west is a point that matches a specular surface 902 within the perpendicular (Y-Z axial plane), it is a point inside an activated layer 903 of the semiconductor laser element 901, that is, a point at a certain depth from the specular surface 902 into the resonator within the composition plane (X-Z axial plane).
Therefore, the converging point of the oscillated beam differs between the composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane), and thus an “astigmatic difference” 904 in optical terms is produced.
When an astigmatic difference occurs, the beam spot is distorted in to a flat, vertically or horizontally oblong spot. Therefore, the beam spot spans mutually neighboring recording tracks of an optical disc, causing a problem of deteriorating a signal characteristic.
It is for this reason that in FIG. 14, the parallel plate 803 is placed inclined at a predetermined angle in the reverse direction in order to correct the astigmatism of the light beam radiated from the semiconductor laser 801.
Moreover, another method proposed to correct such astigmatism of a light beam is canceling out the astigmatism of the light spot by inserting a cylindrical lens in the same optical path of the laser beam.